Refrigerated freezers (hereafter, simply freezers) are ubiquitous in household, commercial and industrial applications. Freezers provide a cold zone of temperature below the freezing point of water at standard pressure irrespective of the ambient temperature outside the freezer. Refrigeration is provided by a refrigeration apparatus, most commonly electrically powered, but other sources of power may be utilized, such as natural gas.
In the event the source of power is lost (a power outage), as for example the result of an interruption of service from a public utility, or even something so local as a child accidentally kicking out a power cord from its socket, the refrigeration apparatus of the freezer stops providing cooling of the cold zone. Heat from the ambient external environment is omnipresent and, assuming this is above the temperature of the cold zone, unremittingly transfers heat into the cold zone through the freezer walls (inclusive of door, seals, etc.). Without a regular extraction of heat by the refrigeration apparatus, heat transfer from the external environment will warm the cold zone, increasing the temperature thereof until the temperature of the cold zone and that of the external environment are equalized.
The rate at which the cold zone will warm depends upon the difference in temperature between the cold zone and the external environment, the thermal conductivity of the freezer walls (inclusive of the door, seals, etc.), the specific heat of the materials of the freezer and the specific and latent heats of objects within the cold zone. The latent heat of the objects depends upon the mass of the objects and the inherent heat of fusion thereof. The combination of these physical properties shall be hereinbelow referred to as a “freezer thermal cross-section”.
A freezer cold zone that is empty (filled with air) will warm much faster than one filled with objects. An example in everyday life of how latent heat retards heating is the duration for which an ice cube remains floating on a glass of tea in the summertime. Concomitant in this example, as long as the ice cube remains, the tea will stay cooler than the external environment of the glass.
While a power outage may be brief and the latent heat of the objects in the freezer able to absorb incoming heat so as to maintain substantially the below water freezing temperature of the cold zone of the freezer, an extended duration of power outage will eventually render a thaw of the objects in the freezer.
In the above nonlimiting scenarios, the power company will eventually restore power or a child may notice days later the plug had been kicked out of its socket and nonchalantly plug it back in. With power restored, the refrigeration apparatus of the freezer will re-establish the below water freezing temperature of the cold zone. A user of the freezer would likely be unable to know whether objects in the freezer stayed frozen or thawed and refroze. This is a worry for those who store frozen foods in a cold zone of a freezer and do not wish those foods to thaw and then refreeze.
Accordingly, what is needed in the art is a thaw indicator for indicating a thaw episode of a cold zone of a freezer has occurred, and further, indicating whether the thaw episode has been followed by a refreeze event.